In FMR experiments, the magnetization precession exhibits a coherent phase of the spin field just like the electromagnetic
field does in a LASER. In this case, the entire spin field shows synchronous dynamics and can be represented by using a
single spin that is well pictured by a classical vector. A quantum state like that is formally described by a coherent state,
which was initially used to derive a fully quantum model of the radiation fields [1]}, [2]} as well
as the coherent behavior of magnons [3]}, [4]}, [5]}. It is
well-known that coherent states represent the more classical-quantum state, i.e., states with minimum uncertainty [6]}.
Consider, for example, a particle in a harmonic potential and represented by a coherent state. In this case, \(\Delta x\Delta p=\hbar /2\) ,
while the wave function describes a dispersionless wave packet that moves harmonically around the minimum of the potential.
Similar semi-classical behavior is reached for the spin field in the FMR. Then, we represent the spin by the phase
angle \(\varphi \)  around the z-axis and the conjugate momentum associated, namely \(S^z\) . In some cases, \(S^z\)  is aligned
with the magnetization direction, but this is unnecessary, Here, we define \(S^z\)  as a transverse component throughout the text,
while the magnetization is along the x-axis. From the classical point-of-view, the fields satisfy the Poisson bracket
\(\lbrace \varphi _i,S_j^z\rbrace =\delta _{ij}\) , and the quantization is achieved by promoting the fields to operators that obey the
commutation relation \([\varphi _i,S_j^z]=\delta _{ij}\) . Similar to the particle case, the operators obey \(\Delta \varphi \Delta S^z=1/2\) ,
which justifies the semi-classical magnetization behavior of the spin. Therefore, it is natural to adopt \(\varphi \) 
and \(S^z\)  as the fundamental operators for describing magnetic models in FMR experiments instead of the usual bosonic representations.
